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Abstract

For powers exceeding a sharp threshold in the vicinity of several hundred watts the beam quality from some narrow bandwidth fiber amplifiers is severely degraded. We show that this can be caused by transverse thermal gradients induced by the amplification process.

Figures (12)

Fig. 1 Light in mode one, traveling to the right encounters a wedge of higher refractive index. The wedge induces a phase tilt which causes the light to oscillate up and down with a period determined by the difference in propagation constants for modes one and two.

Fig. 2 A properly spaced array of index wedges coherently transfers light from mode one to mode two, leading to large amplitude oscillations, or equivalently to large power transfer from mode one to mode two.

Fig. 4 Like Fig. 3 except there is a frequency offset of the light in modes one and two. Because (ω2 < ω1) the irradiance grating moves downstream, and the time lag of the induced temperature profile caused by thermal diffusion makes the refractive index pattern lag the irradiance profile.

Fig. 6 Irradiance profile (left) and temperature profile (right) at the input end of the amplifier. The powers are 1200 W pump, 30 W mode one, 10 W mode two, and the frequency offset between modes is 2 kHz. In this illustration we use an exagerated mode two power to dramatize the oscillations of the irradiance and temperature profiles. The phase delay of the temperature relative to the irradiance is readily apparent in the movie (
Media 1).

Fig. 11 Small signal gain of mode two at 2 kHz versus z for the test amplifier. The solid curve is total gain of mode two and the dashed line is the contribution of laser gain. The mode coupling gain is the difference between the two curves.

Fig. 12 Net gain of thermal mode coupling versus pump power. The symbols are the integrated small signal gains of mode two for three pump powers. The dashed line is for comparison with a linear dependence on pump power.